The present invention relates to a time ratio control power system and, more particularly, to a time ratio control system for controlled energy commutation.
Performance requirements for electrically propelled vehicles, particularly torque-speed requirements, generally dictate the use of direct current (d-c) motors. The d-c motors may be any of the well known type of d-c motors such as, for example, series wound, compound wound or separately excited. Conventionally the armature winding of the d-c motor is energized by direct current controlled by resistors in series circuit arrangement with the motor. By selectively adding or subtracting resistance, the amount of power applied to the motor, and thus the motor performance, may be controlled. As is well known such a control system, although relatively simple, suffers from numerous maladies, the most onerous of which is inefficiency. For this reason time ratio control system or "choppers" have been substituted for series resistors in the more recently developed motor control systems.
In a series chopper control system a controllable switch is located between a source of d-c power and the motor armature, the controllable switch replacing the previously used resistor control. By cyclically opening and closing the switch, pulses of current are metered to the motor. During periods when the switch is open armature current can continue flowing through a free wheeling diode connected across the armature. The armature windings generally act as a large inductive load and tend to smooth the pulsating current into an average current, which determines motor torque. For lower inductance motors a smoothing reactor is connected in series with the motor. Because the switch is either opened or closed, power consumption is primarily in the energy required to open and close the switch and the energy utilized by the motor, although it should be noted that known switches are not ideal and thus some power is expended in the switch itself.
In the present state-of-the-art chopper control systems, the controllable switch is commonly a thyristor or silicon controlled rectifier (SCR). The SCR is a three-terminal device having anode, cathode and gate terminals. When the SCR is forward biased, i.e., the anode terminal is at a positive potential with respect to the cathode terminal, a current signal applied to the gate terminal will cause the SCR to be gated into conduction and to exhibit a negligible anode to cathode resistance. Once gated or fired in this manner, the SCR can only be turned off by subsequently reducing the current through the device to zero and then applying a reverse bias from anode to cathode for a time period sufficient to allow the SCR to regain its forward voltage blocking ability. In practical applications the SCR can be turned off by means of a "commutation" circuit connected in parallel therewith. A detailed description of SCR devices, chopper circuits and commutation circuits may be had by reference to the SCR Manual, Fifth Edition published in 1972 by General Electric Company, Semiconductor Products Department, Syracuse, N.Y.
A typical chopper commutation circuit is a "ringing" circuit, i.e., the circuit contains inductive and capacitive components which develop an oscillating or ringing current. A chopper commutation circuit may include, for example, a capacitor, an inductor, several diodes and a commutating SCR. The chopping frequency is determined by the frequency at which the motor-current conducting main SCR and commutating SCR are fired, and the duty factor is determined by the percentage of a period between consecutive firings of the main SCR that has elapsed when the commutating SCR is fired.
Each component in the commutating circuit must be sized, or rated, to meet the particular current requirements of the circuit application and also the duty factor of the chopper circuit. The actual rating of each component is dependent upon the total energy expended in the circuit, or more precisely, the rating is a function of the watt-seconds per pulse multiplied by the frequency or number of pulses per second. The rating of each of these components determines not only the cost of the commutation circuit but also the weight, efficiency and physical size of the circuit.
Since the function of the commutation circuit is to turn off the main load current carrying SCR, i.e., the controllable switch, a primary design consideration is to assure that the commutation circuit has the capability to perform this function. The capability of the commutation circuit is measured by the magnitude of current which it can supply. In order to commutate an SCR, the commutation circuit must be capable of supplying a current of magnitude greater than the average load current for a time period sufficient to allow the SCR to regain its forward blocking ability. Peak commutating current may be defined in mathematical form as: I=E.sqroot.C/L where I is the current capability, E is the voltage applied to the capacitor, C represents the size of the capacitor in farads and L represents the size of the inductor in henrys. Since commutation capability is proportional to peak commutating current, it becomes apparent that commutation capability can be varied by changing the voltage E, the capacitance C and the inductance L.
In a motor drive system electrical power may be supplied by a prime mover driven generator on board the vehicle or by an external source through a catenary, third rail or trailing cord arrangement. Regardless of the source of power, the voltage supplied by present day systems tends to vary as a function of load current. A typical system, for example, may experience a 2-to-1 voltage variation between low current and high current conditions, with the lowest voltage often occurring at the time when load current is at a maximum. In order to compensate for this voltage variation, it has been necessary to design commutation circuits to commutate rated load current at the lowest expected voltage. Such design techniques have resulted in the use of commutation circuit components of relatively large size, and the commutation circuit tends to be as large or larger than the main load current carrying components. Furthermore, the commutation circuit which supplies rated load current at half-voltage could commutate twice load current at full voltage. For example, a commutation circuit designed to commutate 2000 amperes at 1000 volts could commutate 4000 amperes at 2000 volts. In the typical vehicle system, the source voltage of 2000 volts could occur at low load current conditions, i.e., when the main SCR is supplying an average load current less than 1000 amperes. Accordingly, the commutating circuit must also be capable of dissipating the excess energy losses associated with the higher commutation circuit currents at 2000 volts.